Prior use of autofluorescence endoscopy has been limited to fluorescence in the visible spectrum with unselected contributions from a number of fluorophores, both cellular and extracellular. Fluorescence has not been so stratified by excitation wavelength to consider the roles played by individual fluorophores. Imaging of cellular fluorophores such as tryptophan has been slowed by limited availability of UV-capable microscope objectives and sub-300 nm light sources.
White light colonoscopy is the preferred screening technique for colon cancer but fails to detect a significant number of polyps and flat neoplasms. Improving the detection rate can help prevent incident cancers and decrease screening intervals, thus improving screening effectiveness while reducing the overall cost. Low contrast lesions (LCLs), including flat lesions such as those having a height less than half their width, are of special concern because they are more frequently cancerous than polypoid lesions. In a recent study, flat cancerous lesions were present in 9.4% of male veterans undergoing colonoscopy. Indigo carmine chromoendoscopy increases the detection of flat neoplasms, but is time consuming for use during screening exams in which a high volume of colonoscopies must be performed under demanding time constraints.
Identifying flat lesions using an endogenous contrast mechanism such as autofluorescence (AF) provides benefits over chromoendoscopy by reducing exam time and eliminating dye toxicity concerns. AF image contrast in tissue is derived from native tissue fluorophores (such as tryptophan, collagen, NADH and FAD) as well as from the effects of absorption and scattering of other components (e.g., hemoglobin). Spectroscopic AF studies comparing normal and neoplastic tissues have consistently noted reduced AF intensity from neoplasms. This general result was obtained in colon tissue at excitation wavelengths including 337 nm, 370 nm, and 442 nm. Discrimination based on reduced AF intensity has put emphasis on correcting AF intensity measurements for variations in absorption, illumination intensity, and tissue surface morphology. It has also drawn attention to methods such as time-resolved AF imaging, which largely avoids the confounding factors for intensity measurements but has its own drawbacks including instrumentation complexity and long acquisition times.
Commercial AF endoscopes for the colon include the AFI system (Olympus Medical Systems, Tokyo, Japan) and PINPOINT system (Novadaq Technologies, Mississauga, ON, Canada; formerly the Onco-LIFE and LIFE-GI system, Xillix Technologies). The first developed was LIFE-GI which illuminated tissue with blue light (400-450 nm) and measured both green AF (490-560 nm) and red AF (>630 nm). Later generations of LIFE-GI have included simultaneous blue and red illumination and ceased collection of red AF in favor of red reflectance (>630 nm). The AFI system illuminates with blue light (395-475 nm) and green light (540-560 nm) in succession and measures green/orange AF (490-625 nm) followed by green reflectance. Both of these commercial endoscopes electronically combine a blue-excited AF image and a reflectance image in a single pseudocolor image presented to the physician. Color differences in the pseudocolor composite image are used to signal the observer to the presence of a lesion. Contrast in images from these endoscopes is produced primarily by a loss of green AF in lesions and is perceived as a changed color ratio. Intensity artifacts due to geometrical shape of the specimen would be apparent in a single image; however, they are reduced in the pseudocolor image because the AF and reflectance images are affected by the same artifacts, preserving the color ratio of the resulting image. Several randomized trials comparing AF endoscopy in the colon to standard video endoscopy, narrow band imaging (NBI), or high resolution endoscopy have been published very recently. The outcomes of these studies have been mixed, with some indicating commercial AF endoscopes can reduce polyp miss rate and others showing no significant improvement of AF over other technologies.
Narrow Band Imaging® (NBI, Olympus Inc. New Hope, Pa.) uses blue and green light that is avidly absorbed by hemoglobin and displays blood vessels with high contrast and enhances the visualization of superficial mucosa. Flexible Spectral-Imaging Color Enhancement (FICE®, Fujinon Inc., Wayne, N.J.) uses white light illumination followed by spectral estimation to produce high contrast images. However, neither NBI, FICE or I-Scan® (Pentax Medical Co, Montvale, N.J.) have been shown to improve the detection rate of neoplasms compared to high resolution white light endoscopy.
When light illuminates the mucosa, it is largely reflected and scattered. Some of it is absorbed and re-emitted at a longer wavelength by molecules in tissue (fluorophores) to produce fluorescence of a redder color than the illuminating beam. Fluorophores are inherent biological compounds that emit light, most notably metabolic co-factors such as NADH and FAD, amino acids such as tryptophan, structural proteins such as collagen and elastin as well as porphyrins. Early measurement systems relied upon broadband autofluorescence with unselected contribution from NADH, FAD, collagen and elastin. Initial work with fiberoptic instruments showed reduced fluorescence with neoplastic change. Auto-Fluorescence Imaging (AFI) (Olympus Inc. New Hope, Pa.) is an endoscopic autofluorescence system using blue light excitation in the 400-500 nm wavelength range to produce autofluorescence at 490 to 625 nm. A reflectance image of the mucosa is then taken with green light (550 nm). A pseudocolor (magenta) is computed to show the areas of decreased fluorescence and the surrounding normal mucosa appears green from the reflected light, with the blood vessels appearing dark green. The Onco-Life system (Xillix Technologies Corporation, Richmond, BC, Canada) uses blue light (400-450 nm) for excitation, captures fluorescence from 490 nm to 560 nm and combines it with a red reflectance image. The results from the existing autofluorescence endoscopes have been mixed, with some showing increased detection of polyps, while others showed no improvement over white light endoscopy with missed detection of flat lesions.
Techniques such as enhanced backscattering spectroscopy, partial-wave spectroscopic microscopy and karyometry can be used for risk stratification but are still dependent on standard white light colonoscopy for the detection of neoplasms.
A need exists for optical techniques and instrumentation that sufficiently enhances the image contrast of LCLs, so that the latter can be easily seen and not missed, even during a busy endoscopy schedule or at the end of the queue. The ideal solution preferably highlights the presence and location of a neoplasm, including those that are difficult to see with the naked eye, without dependence on labels or other exogenous chemicals. There is also a need for a technique that highlights the presence of neoplasms conveniently such as by turning on a switch. Finally, a need exists for a multispectral imager with UVC excitation and detection capability, including sub-300 nm excitation.